In certain modern roofing installations of commercial and factory buildings having a flat roof design, a layer of insulation is placed on a generally corrugated steel roof deck, and is then covered with a single ply thermoplastic roofing membrane to protect against the elements. Conventional membranes are EPDM, PVC or equivalent materials. Conventional insulation of the type used for roof decks includes ISO, wood fiber board, or pearlite.
The assemblies typically used to secure the insulation generally include a washer-like stress plate made of either plastic or metal and which receives a screw-like fastener that is threaded into the roof deck, thereby clamping the insulation between the stress plate and the roof deck. Since the type of insulation commonly utilized is approximately up to six or even twelve inches thick, it is important to keep the fastener in perpendicular alignment when installing the plate so that the stress plate properly contacts and secures the insulation against blowing off from extreme cyclical loading. Adverse weather conditions such as hurricanes and other storms having high and gusting winds create the extreme high pressure dynamic loading, including uplift, of the sort which such roofing is designed to withstand. In practice, special tools may be used to install the washers and fasteners. An example of such a tool is described in U.S. Pat. No. 4,809,568, and another such tool is sold by ITW Buildex, Itasca, Ill. under the trademark ACCUFAST.
In a typical installation, the stress plates and fasteners are delivered to the job site in separate packages. Next, the installer obtains a supply of plates and fasteners, and lays out the plates on the roof in a specific pattern required by the roofing approval or warranty. Upon completion of that step, the installer installs fasteners through the plates, into the insulation and/or membrane, and eventually into the solid roof base or substrate. In some applications, one type of plate and fastener is used to secure the insulation, and another type of plate and fastener is used to secure the membrane.
A disadvantage of this method of installation is that it requires the installer to spend a significant amount of time to perform the two major steps of separately handling the stress plates and the fasteners. Also, once installed, the alignment of the fasteners relative to the plates and the roof must be maintained under relatively exposed working conditions. Often it is difficult to prevent the fasteners from being threaded into the roof at an angle, which may decrease their ability to secure the roof when exposed to severe weather.
Another disadvantage of conventional stress plates and their associated fasteners relates to the anti-corrosive coating which covers typical fasteners. In some cases, the corresponding hole or aperture in the stress plate is dimensioned so that a tight fit is formed with the fastener. Thus, when the fastener is threaded into the hole, the sharp edges of the hole tend to scrape off or otherwise rupture the anti-corrosive coating. In time, upon exposure to the elements, the fastener corrodes more rapidly, and eventually fails, causing the roof to be more susceptible to severe weather damage.
Another disadvantage of conventional stress plates is that upon installation on a roof, when the roof system is subject to severe weather conditions, such as high winds, the roof insulation may billow or pull on the fastener assembly, creating a stress load. Such stress loading may also affect the plates securing the insulation. In extreme cases, the stress loading has been known to pull the plate over the head of the fastener. Naturally, this type of plate deformation is to be avoided, since it may ultimately lead to failure of the roofing system.